The present invention generally relates to vacuum pumps, and more particularly, to pumps known as turbomolecular pumps characterized by "bladed" rotor and stator construction with running clearances in the millimeter range which are particularly effective in the free molecular flow range. More particularly, this invention is directed to an improved bearing system for vertical rotating shafts and is especially useful for vacuum pumps of the single ended type which have an upper bearing between the motor and the pump rotor discs. In this regard, an important aspect of the present invention concerns a bearing assembly for a single-ended turbomolecular pump which is characterized by improved heat transmission capabilities and a high tolerance for misalignment whereby noise, vibration and wear are minimized and longer bearing life is achieved.
In conventional single-ended vertical turbomolecular vacuum pumps, two vertically disposed bearings are commonly employed to support the rotating system with the mass center of the rotating system usually being concentrated slightly below the upper bearing. Maintaining stability in rotating machines of this type requires great care due to the very high rotational speeds which are utilized in these machines, particularly in the order of 40,000 to 50,000 r.p.m. or higher. Replacing worn bearings in these machines involves the disassembly of the rotating system which by itself is a laborious and time consuming process. Unfortunately, because the natural spin axis is slightly different with each bearing replacement reassembly, the rotor system can never be in perfect balance on its bearings. Ill-balanced bearings can contribute to eccentric rotation of the pump shaft and intolerable and undesirable vibration. A sufficiently compliant bearing mount is therefore required for bearing replacement by the user without rebalancing facilities.
In addition to supporting the rotating system, bearings also serve to conduct heat away from the rotor. Most commercially available turbomolecular pumps employ rotors of aluminum alloy which are subject to loss of strength starting at about 148.degree. C. Since turbomolecular pump rotors operate in a thermally insulating vacuum, the only way the rotor can be cooled, without unduly complicating the system, from common heat input such as bearing friction, integral motor losses and gas friction, is by conduction through the bearing lubricant and by radiation. It is therefore critical to provide adequate bearing cooling in high speed machinery.
Heretofore, the majority of turbomolecular vacuum pumps have utilized oil lubricated ball bearings. Other types of bearings such as air or gas film bearings and magnetic bearings have also been employed but have not achieved widespread popularity for a variety of reasons. For instance, the low stiffness provided by magnetic bearings and the complexity and expense of the electronic control systems necessary to maintain them result in operational limitations and reliability problems.
Ball bearings are well adapted to relatively high speed applications because their friction increases only moderately as the speed increases, and they do not generally develop internal instability with increasing speed. Furthermore, they offer a great reserve of strength and rigidity to handle extraordinary air inrush thrust loads. These bearings are located at the discharge side of the rotor disc cascade, and oil vapors associated therewith do not penetrate upstream to the turbo inlet.
However, while the reliability of the ball bearings currently available is very high, they have certain inherent deficiencies which severely limit their use in high speed turbomolecular pumps. For example, the balls orbiting around the axis at very high angular velocity cause substantial contact forces between the balls and raceway which are far greater than those imposed by the rotor alone. Misalignment of the bearing raceways relative to the rotor axis converts these forces to undesirable noise and wear which eventually requires replacement of the bearing. Additionally, ball bearings typically rely upon a ball separator, which if ruptured in a failure of the bearing imparts a dissymetry to the bearing assembly which may permit the rotor to tilt and contact the stator. At high rotational speeds, this contact may lead to catastrophic failure of the pump. Moreover, ball bearings, when misaligned are susceptible to extreme wear which eventually requires replacement of the bearing. Precise machining and meticulous care in handling and assembling the parts are, therefore, a necessity to minimize misalignment.
In addition to the handling and assembly constraints, the heat transmitting capabilities of ball bearings limit the effective operating range of a turbomolecular pump. Two principal sources of heat input to a turbomolecular pump rotor system are oil friction and gas friction. The gas friction heat input is proportional to the pressure range in which the rotor is operating, i.e. lower ga friction at lower pressures and higher gas friction at higher pressures. The heat generated from gas friction is usually dissipated by the flow of oil through the bearing supporting the rotor. However, the flow of oil required for a typical ball bearing assembly is proportional to both its heat transmission capability and the amount of friction generated by the oil itself.
Therefore, when gas friction heat input is negligible, a ball bearing with minimum oil is best while when gas friction heat input is high, maximum oil is best. However, the concomitant effect of increased oil friction with increased oil throughout eventually produces a counter-balancing effect to the heat transmission capability of the bearing which limits the high pressure end of the useful range of the pump and thereby it curtails the use of turbomolecular pumps in the higher pressure range often needed for sputter and dry etch applications used in integrated circuit manufacture.
At first glance ordinary sleeve-type bearings appear to present a reasonable alternative to the use of ball-bearings in high-speed rotational machinery. However, it has been widely recognized that the use in vertical rotating machinery of sleeve-type bearings which rely upon oil-film lubrication between the rotating shaft and bearing sleeve is limited by the unusual vibrations which commonly occur at high rotational speeds. These vibrations, which have been referred to in the art as "oil whip", "oil whirl", "shaft whip" or "shaft whirl" are caused by a pumping effect which the shaft has on the oil which increases the oil pressure at some points more so than at others around the circumference of the shaft. This uneven oil pressure distribution exerts a force on the shaft and causes the shaft to maintain an eccentric rotation within the sleeve. Self-excited vibrations of undesirable magnitude are set up, which are commonly referred to as pump noise. It has been further recognized in the art that when the angular velocity of the oil, .omega..sub.o is equal to or smaller than half the angular velocity of the shaft, .omega..sub.v, the oil whip develops (i.e., if the shaft runs faster than twice its critical speed). The efficiency of the pump is severely reduced due to these vibrations. Therefore, it is desirable to provide a sleeve-type bearing for high speed rotating turbomolecular pumps that significantly reduces the vibrations due to oil whirl.
The thermally insulating vacuum present in the operational environment of turbomolecular pumps also presents a limitation on the use of conventional bearings. The only significant points of contact for transfer of heat generated by friction from the collisions of gas molecules on the rotor blades is the surfaces of the pump bearings. Excessive heat applied to the bearings can cause both a high-temperature breakdown of the lubricant and bearing failure. Therefore, it becomes necessary to provide an adequate oil flow to the clearance between the bearing and the journal. However, the flow of oil supplied to this clearance is important in that excessive oil can create hydrodynamic friction and turbulence in the oil. Therefore, if the oil flow is too great excessive lubrication friction occurs and the bearing lubrication tends to introduce heat into the system as opposed to removing it.
Attempts have been made to correct these problems and have included the use of specific bearing surface configurations such as interrupted bearing surfaces, or arcuately formed "lands" which are separated by axially extending recesses, wherein oil is supplied between the shaft and the bearing at the recesses. See, for example, U.S. Pat. No. 4,427,309. Such a bearing is specifically designed to "float" or rotate within its housing at a speed generally less than that of the shaft. These recesses not only absorb the excess oil pressure formed by oil whirl but also tend to introduce turbulence into the oil flow which increases the heat input due to lubricant friction. An extra oil film and delivery system must also be provided between the bearing outer surface and the housing. This additional oil introduces additional lubricant friction to the bearing. Furthermore, the overall length of such a bearing must be increased over that normally used for such applications because the recesses reduce the load bearing surface of the bearing.
The present invention departs from the prior art and solves the aforementioned problems by providing an improved bearing assembly of the "plain" or "sleeve" liquid lubricated type which can tolerate far more misalignment than a typical ball bearing without increase in noise, wear, or fatigue damage and which has better heat transmission capabilities than a typical ball bearing. Heretofore such bearings have not been employed in turbomolecular pumps because such bearings having a conventional length to diameter ratio would impose far too much fluid friction drag at the speeds employed in such turbomolecular pump.
It has been discovered that when the ratio of journal diameter to bearing length is relatively high, 10:3 or above, the above noted disadvantages are diminished to the point where the frictional drag encountered is less than that obtained with a ball bearing of comparable diameter and a lubricant of comparable viscosity and where there is no appreciable whirl. In addition, when the bearing utilizes a relatively smooth and uniform inner working surface and the lubrication passageways are generally disposed along the center of the journal and the bearing, the oil supplied to the clearance forms a symmetrical oil field therein. A hydrodynamic pressure develops in this oil field which is sufficient to maintain separation of the journal and the bearing surfaces. Consequently, eccentric rotation of the shaft due to oil whirl and the undesirable vibration which accompanies oil whirl virtually disappears and bearing speeds in excess of what were previously attainable in vertical turbomolecular pumps are achieved. In addition, this new bearing assembly has the potential for unlimited life and therefore avoids the meticulous balancing problems of bearing replacement or reassembly.
It is, therefore, a general object of the present invention is to provide an improved turbomolecular pump.
Another object of the present invention to provide a turbomolecular pump with a improved bearing assembly which substantially reduces noise, wear or fatigue damage.
Another object of the present invention is to provide a turbomolecular pump for operation in the free molecular flow range with an improved journal bearing assembly which has improved heat transmission capabilities thereby extending the useful high pressure range of the pump.
It is a further object of the present invention to provide an improved journal bearing assembly for use on vertical turbomolecular pumps which has two journal and two bearing working surfaces.
It is yet another object of the present invention to provide a turbomolecular pump with an improved lubricated journal bearing assembly wherein the lubricant is supplied to the bearing through the bearing and the journal.
Another object of the present invention is to provide an improved turbomolecular pump of the single ended type which incorporates an oil film journal bearing assembly in which a uniform hydrodynamic pressure field is established within the journal-bearing clearance.
Another object of the present invention is to provide an improved turbomolecular pump which utilizes two different oil feeding systems to provide lubrication to the pump.